The present invention refers to polyolefins functionalized with maleic derivatives, their preparation, and their use in the field of compatiblization of polymeric compositions.
Olefin polymers and copolymers modified by functionalization of unsaturated monomers have been known for some time and have been used mainly as impact modifiers in polyamide compositions (impact-resistant agents), as modifiers of the viscosity of lubricant oils, or as adhesive components to be interposed between materials which are incompatible with one another.
In the field of reinforced materials, maleic anhydride is the most widely used functionalizing monomer. Examples of polymers functionalized with maleic anhydride are EVA, polyethylenes having different densities (LLDPE, VLDPE, ULDPE, HDPE), EPR, EBR, EPDM, etc. Also known are polyamide-reinforced compositions containing the said polymers as impact-resistant agents.
Functionalization or polyolefins is usually made via addition of radical initiators to mixtures of polyolefin and a functionalizing monomer, either via a continuous process in suitable extruders, or via a non-continuous process in closed mixers.
To increase the compatibilizing power of functionalized polymers, attempts have been made to increase the percentage of functionalization. However, in the case of polyolefins functionalized with maleic anhydride the functionalization yield is not high and involves a considerable residue of non-reacted anhydride. To increase the percentage of functionalization it is therefore necessary to use sophisticated plants, with an accurate control of temperature, a high number of degassing positions, as well as safety devices and devices for controlling environmental pollution (cf. Reactive Extrusion: Principles and Practice, Ed. M. Xanthos Hanser, pp. 10-16). Maleic anhydride in fact has a considerable volatility (sublimation starting from 53xc2x0 C.), and its dispersion in the environment is a source of toxicity. The maximum allowable concentration (MAC) in. air for maleic anhydride is 0.25 ppm. In addition, the increase of the percentage of maleic anhydride decreases workability (cf. Makromol. Chem., 1989, 190(6), 1457), thus requiring further steps to ensure uniform incorporation in the polyamides.
The alternative solution, consisting in increasing the level of radical initiator, is not satisfactory, in that it leads to a corresponding increase in undesired phenomena of cross-linking, with formation of gelated products (cf. Reactive Extrusion: Principles and Practice, Ed. M. Xanthos Hanser, pp. 10-16).
The presence of products of cross-linking and gels constitutes a serious limit to the use of functionalized polyolefins. These by-products, in fact, modify the rheology and uniformity of behaviour of the polyolefins, thus rendering their compatibilizing effect irregular, with possible tearing of films, alteration of the functional performance and features of reinforced polymers, or decoupling between materials adhering to one another.
In EP 581360, the use of extruders with a high mixing capacity, associated to the use of extremely pure ethylene copolymers, has enabled a greater homogenization of the initiator and a lower degradation of the product. The polymers are produced in an extruder by feeding in maleic anhydride diluted in high-density polyethylene. However, the values of functionalization with maleic anhydride have been found to be lower than 0.2%.
In U.S. Pat. No.4,762,890, to melted polyethylene, vehicled inside a double-screw extruder, is added a solution in polar solvent containing maleic anhydride and a peroxide initiator. The functionalizing agent thus solubilized is more dispersible, but on the other hand the polarity of the solvent limits the homogenization of the maleic anhydride in the polyethylene hydrophobic phase. In this case, the functionalization values have been found to be lower than 1%.
Polyolefins with a high degree of functionalization have been obtained using diethyl maleate as functionalizing agent, instead of maleic anhydride. However, problems have been encountered as regards uniform mixing and presence of products of cross-linking and degradation (cf. J. Serb. Chem. Soc., 56, 1991, 191).
Lately it has been possible to create polyethylenes functionalized with diethyl maleate with a high degree of functionalization and a low degradation by working in extruders or Brabander mixers and operating with a precise maleate/radical initiator ratio (cf. Gazz. Chim. Ital., 1977, 127, p.1). Also this solution is not exempt from drawbacks: in fact, given the greater reactivity of the maleate with respect to the anhydride, it is necessary to work in reduced times, in extremely efficient conditions of mixing, and within restricted quantitative ratios between the reagents used.
EP 137 940 describes a process to prepare polyolefin rubbers modified with maleic anhydride and one or more di C1-C10 esters of maleic or fumaric acid; this document does not disclose how to prepare compounds with functionalization degrees comprised between 2% and 20%; no specific compounds having the above functionalization degrees are disclosed and/or prepared in this document; according to this document polyolefins grafted with maleic anhydride contain gels in amounts up to 3% by weight.
All these attempts bring out the impossibility of achieving high levels of functionalization in an efficient way in the case of polyolefins functionalized with maleic derivatives. Such products prove obtainable only at the expense of adopting costly adaptations of the process and working in restricted windows of operating conditions. There thus remains unresolved the problem of obtaining, in high yields and without impurities, polyolefins that are highly functionalized with maleic derivatives. There is likewise felt the need for bringing about functionalization in an industrially simple way and with wide margins of operating conditions.
Described herein are polyolefins functionalized with maleic anhydride and at least one high-boiling ester of the latter, and a process for their preparation in high yields, with high degrees of functionalization, without the production of by-products of cross-linking or degradation. The process, which may be carried out either continuously or discontinuously, is characterized by the use of inorganic substances having a specific porosity, grain size, density and humidity, as vehicles of the functionalizing monomers; the presence of maleic ester contributes to enhancing the reaction efficiency. Likewise described herein is a double-screw extruder devised for carrying out the said process, and the use of products obtained as compatibilizing additives in the field of adhesive and reinforcing polymers.